Antenna



July 26, 1949. R. J. ADAMS ETA]. v I 2,476,949

ANTENNA Filed Aug. 2, 1945 1 ROBERT -J.- ADAMS HARVEY W. LANCE Patented July 26, 1949 ANTENNA Robert J. Adams and Harvey W. Lance, \Vashington, D. C.

Application August 2, 1945, Serial No. 608,549

8 Claims.

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 O. G. 757) This invention relates to wide-band antennas.

An object of the invention is to provide a Wideband antenna operative at substantially halfwave resonance including means effecting an impedance match between said antenna and a coaxial transmission line without the use of an external balance-to-unbalance transformer or a physical ground plane.

Another object of the invention is to provide a novel wide-band directional antenna array.

In accordance with the invention, the wideband antenna comprises an elongated member divided asymmetrically into two sections and a coaxial transmission line entrant into said member at the longitudinal center thereof. The conductors of the line are expanded to intersect the surface of the elongated member in planes perpendicular to the longitudinal axis thereof at a distance less than one-quarter wavelength from one end of said member. A metallic link connecting the ends of the two sections provides mechanical rigidity. The effective shunt capacitive reactance between the expanding portions of the line is made anti-resonant with the inductive reactance of the link, thereby providing a substantially non-reactive termination for the line, the impedance of the termination bein equal to the input resistance of the antenna and to the characteristic impedance of the line.

Additionally, the invention provides a directional antenna array comprising a plurality. of antennas as hereinbefore described and a plurality of parasitic antennas of similar shape.

A more complete understanding of the invention may be had by reference to the following detailed description of an exemplary embodiment and the accompanying drawings in which:

Fig. 1 is a side view of a dipole antenna in accordance with the invention,

Fig. 2 is a sectional view in the plane 2-2 of Fig. 1,

Fig. 3 is a vertical section of Fig. 1, and

Fig. 4 is an isometric view of a directional antenna array in accordance with the invention.

Referring now to Fig. 1, there is shown an antenna in accordance with the invention comprising two colinear metallic sections ll, l2 of unequal lengths, an insulator l3 between sections l I, I2 and a metallic link I4 conductively attached at each end. to sections I l and 12, respectively.

A coaxial transmission line 15 is joined to section i l at a position midway between the extreme ends of the antenna, the diameter of said line iii being less than the diameter of the sections l i, 52. The ends of the antenna may be rounded oif in hemispherical form as shown.

The electrical properties of an antenna are dependent on its physical dimensions and shape. Consequently, it is desirable that the antenna be suiiiciently rigid to resist deformation by externally applied forces due to shock, vibration, or other causes. From a consideration of the properties of insulating materials known to the art, it will be found that the mechanical strength of said materials is, in general, relatively low. The metallic link i4 therefore provides additional mechanical rigidity in addition to that provided by insulator E3 in order that the element I2 may not be displaced appreciably from its position rela tive to element it by external forces. The dimensions of the link id are determined primarily in accordance with the magnitude of the external forces which it is contemplated may be applied to the antenna.

The electrical properties of the antenna of Fig. 1 will now be considered. It was found that an optimum ratio Of diameter D to length L in a preferred embodiment of the antenna is of the order of 0.15 and that an antenna constructed in accordance with said ratio functioned to radiate energy with relatively constant efficiency over a wide band of frequencies. The length L of the antenna is such that half-wave resonance obtains at the center of the band of frequencies for which it is designed. Due to the end effect of radiation and the relative thickness of the antenna, it was found that half-wave resonance occurs when the length L is 0.46 of the Wavelength of the applied energy.

Referring now to Fig. 3 there is shown a sectional View of Fig. 1. The interior of cylindrical sections H, 12 may be hollow as shown in order to reduce the weight of the antenna. Radio frequenc energy applied to the transmission line 15 induces currents which flow through said line on the inner surface of the outer conductor and in the opposite direction on the outer surface of the inner conductor of line l5. The outer surface of the outer conductor of line i5 remains at zero potential because the radio frequency currents in the line l5 are concentrated on the interior thereof, due to skin effect. The line I5 enters the antenna section II at a position of 7 of sections H and i2 respectively. The linear rates of expansion of the two conductors are ex perimentally determined in order that the ex:

panding portions thereof provide effectively-"a capacitive reactance equal to and in parallel with the inductive reactance of the link [4, so

that a parallel resonant circuit is formed which has impedance equal to the resistive component of the antenna impedance.- The insulating ma terial I3 of the line If: divides the antenna into two sections H, I? of unequal lengths. The relative lengths of sections H and I2 determine the antenna impedance at the termination of line It with the surface of sections H, [2. Preferably, the relative lengths of sections it and I? are .adjusted so that the resistive component.

of the antenna impedance is equal to the characteristic impedance of the line l5. VJl-ien the reactive components of thetermination imped ance of line I5, including the inductive reactance of link: 55, are resonant as hereinbefore specified, tl1e effective Q of the resonant circuit is lowbecause the circuit, is effectively in parallel with the resistive component of the antenna ini pedance. Thus; the termination impedance of line it remains predominantly resistive and rel" atively constant over a wide'band of frequencies, thereby enabling the antenna to absorb a rel atively constant amount of energy. Due to the shape of the antenna, and particularly to its large diameter, said antenna functions with relatively constant efficiency to radiate said energy' over awide band of frequencies. V

Dipole antennas constructed in accordance with the invention are well adapted to be used aselernents of a directional antenna array, par

ticularl when said array is required to operate effectively overa wide band of frequencies. For example, Fig. 4 shows an isometric view of a fourelement directional antenna array comprising two coplanar parallel dipole antennas 20, 21 and two coplanar parallel parasitic antennas 22, 23 supported in parallel spaced'relationship'to antennas 28 and 2!, respectively, by rods 24 and.

'25 of equal length. The distance between the planes of the dipole antennas 2d, '2! and the para" sitic antennas 22, 5'3 may be of the order of 0.22 wavelength. The dipole antennas 2t and 2| 1 are spaced one-half wavelength apart by transmission line it, and are oppositely oriented inspacel Consequently, in order to excite". an? tennas 2i! and 2! in phase; a 180 degree phase shift must be introduced in the line l5 conne'cting' said antennas. This phase shift is accom plished conveniently by eifecting a junction 17 between line 23 and line that a position such.

that the phase ofthe'voltage applied through said junction 2? toiantenna 2t lags the'voltage" applied to antenna it byliiil degrees. When the dielectric constantiof the line is greater than a unity, the physical displacement of the junctio'n z'tfrcm the center of line I5 'is less th m one- The line [5 is bent quarter wavelength, as is indicated qualitatively in Fig. 4. When the dielectric constant is unity,

, as with air insulation, the junction '21 should coincide with the junction of the line It with antenna 29. The radiation field of the array of Fig. 4 has two intensity maxima along a line perpendicular to the plane of the dipole antennas '2, 2|. The opposite orientation of dipole antennas 20, 2| minimizes the effect of any residual phase difference between currents flowing in the two portions of each dipole antenna 20, 2| which might otherwise result in an asymmetrical radiation pattern.

An outstanding advantage of the invention lies in the fact that, although an unbalanced concentric transmissionline is used to feed energy I to the antenna, no frequency-selective elements commonly known asffbazookas are required to convert the unbalanced feed to a balanced feed,

the conversion being accomplished within the. antenna itself which has minimum frequency selectivity. 7

It should b understood that the practice of the inventionis; not restricted to the embodi ments illustrated and described but is circum-' scribed only by the scope and limitations of the appended claims. a 1 I 7 The invention described herein maybe manufactured and used by or for the Government of the United States of America for governmental purposes without the paymeht'of any royalties thereon or therefor. j 1 What is claimed is:' w

1. An antenna comprising-an elongated hollow member divided asymmetrically into two sections,

a coaxial transmission line entrant intothe memher at the longitudinal center thereof, and means coupling. eachjc'onductor of the coaxial line to a respective antennasection'.

2. An antenna comprising an elongated hollow member divided asymmetrically into'two sections', a coaxial transmission line entrant intoth'e memher at the longitudinal center thereof and termihating in a line portion coaxial with the longitudinal axis ofthe antenna, and means connect- 7 .ing each conductor of the line portionto ajresp'ece tive antenna section;

0. An antenna 5 'omprisin'g an elongatedhollow member divided asymmetrically into two. sections, a coaxial transmission line entrant into. the memher at thelongitudinal cehterthere'of and terminating in a line" portion coaxial with the longitudinal axis of the antenna, and expanded conductor means connecting the line to the adjacent 7 ends of the sections. i V a a 4. An antenna comprising an elongated hollow cylindrical member divided asymmetrically into two sections, a coaxial transmission line entrant into one section at the longitudinal center of the member and terminating, in'a line portion coaxial withtheicylindrical membe'na pair .of conical conductors connecting the adjacent section ends to the line, and adielectric member. interposed between the, conical conductors' v I 5. An antenna comprising'an elongated hollow Cylindrical hlel'fibei divided asymmetrically! into two sections,'a coaxialtransmission line entrant a into one section at thelongitudinalcenter of-the member and terminating in a line portion coaxial .with the cylindrical member; 'a pair; of jconical conductors;connecting the adjacent sectionle'nds to the line, a dielectric member-linterposedbe: tween the canner conductors, andan inductive a metallic member externally}connecting 'the an- 6. A dipole antenna comprising a cylinder having a radius of the order of 0.035 of the mean operatin Wavelength and a length of the order of 0.5 of said Wavelength, a coaxial transmission line entrant into the interior of said cylinder at the longitudinal center thereof, the inner and outer conductors of said transmission line being expanded to intersect the surface of said cylinder asymmetrically of its length in planes perpendicular to the longitudinal axis of said cylinder, and a metallic link. connecting said portions, the inductance of said link being resonant With the efiective capacitance between the expanded portions of said conductors to provide a substantially non-reactive termination impedance for said line.

7. A directional antenna array comprising a plurality of coaxial parallel elements, each of said elements comprising a dipole antenna in accordance with claim 4, the shorter portion of adjacent elements being oriented in opposite directions relative to the common axis of said elements, and a plurality of cylindrical parasitic elements having substantially the same radii as the dipole elements and coupled thereto by the radiated field of said dipole elements.

6 8. An antenna array comprising a plurality of parallel asymmetrical dipole antennas substantially as described in claim 4, the adjacent dipoles of the array being inverted one from the other and spaced apart by one half wave length.

ROBERT J. ADAMS. HARVEY W. LANCE.

REFERENCES CITED The following referenrces are of record in the file of this patent:

UNITED STATES PATENTS 

